Sealed integral MEMS switch

ABSTRACT

A MEMS switch includes a micro-machined monolithic layer ( 122 ) having, a seesaw ( 52 ), a pair of torsion bars ( 66   a   , 66   b ), and a frame ( 64 ). The frame ( 64 ) supports the seesaw ( 52 ) for rotation about an axis ( 68 ) established by the torsion bars ( 66   a,    66   b ). Shorting bars ( 58   a   , 58   b ) at ends of the seesaw ( 52 ) connect across pairs of switch contacts ( 56   a   1, 56   a   2, 56   b   1, 56   b   2 ) carried on a substrate ( 174 ) bonded to one surface of the layer ( 122 ). A base ( 104 ) is also joined to a surface of the layer ( 122 ) opposite the substrate ( 174 ). The substrate ( 174 ) carries electrodes ( 54   a   , 54   b ) for applying forces to the seesaw ( 52 ) urging it to rotate about the axis ( 68 ). An electrical contact island ( 152 ) supported at a free end of a cantilever ( 166 ) ensures good electrical conduction between ground plates ( 162   a   , 162   b ) on the layer ( 122 ) and electrical conductors on the substrate ( 174 ).

The present invention relates generally to the technical field ofelectrical switches, and, more particularly, to micro-electro mechanicalsystems (“MEMS”) switches.

BACKGROUND ART

Radio frequency (“RF”) switches are used widely in microwave andmillimeter wave transmission systems for antenna switching applicationsincluding beam forming phased array antennas. In general, such switchingapplications presently use semiconductor solid state electronicswitches, such as Gallium Arsenide (“GaAs”) MESFETs or PIN diodes, ascontrasted with mechanical switches. Such semiconductor solid stateelectronic switches also are used extensively in cellular telephones forswitching between transmitting and receiving.

When RF signal frequency exceeds about 1 GHz, solid state switchessuffer from large insertion loss in the “On” state (i.e., when anelectrical signal passes through the switch) and poor electricalisolation in the “Off” state (i.e., when the switch blocks transmissionof an electrical signal). MEMS switches offer distinct advantages oversolid-state devices in both of these characteristics, particularly forRF frequencies near or exceeding 1 GHz.

U.S. Pat. Nos. 5,994,750, 6,069,540 and 6,535,091 all disclose MEMSswitches in which a pair of coaxial torsion bars, a pin or a pair offlexible hinges support respectively substantially planar and rigidbeams or a vane for rotation about an axis established by the torsionbars, pin or flexible hinges. In all three patents, the pair of coaxialtorsion bars, the pin or the pair of flexible hinges respectivelysupport the substantially planar and rigid beams or vane a smalldistance above a substrate. U.S. Pat. No. 5,994,750 (“the '750 patent”)discloses that ends of the torsion bars projecting outward from the beamand anchored respectively to a pair of support members alone support thebeam the small distance above the glass substrate. Both U.S. Pat. No.6,069,540 (“the '540 patent”) and U.S. Pat. No. 6,535,091 (“the '091patent”) interpose respectively the pin or an upper and lower fulcrumlocated at the flexible hinges between the beam or vane and thesubstrate to maintain a spacing therebetween.

In the instance of the '750 patent, the beam extends to only one side ofthe torsion bars so its rotation thereabout in closing an electricalswitch provided thereby is equivalent to the movement of a door swingingon its hinges. Alternatively, both in the '540 and '091 patents therespective beam or vane extends in both directions outward from the pinor pair of flexible hinges. Thus in the structures respectivelydisclosed in these two patents, in closing an electrical switch thebeam's or vane's rotation about the axis established by the pin or pairof flexible hinges resembles the movement of a seesaw. In all threepatents, electrostatic attraction induces rotation which effects switchclosure.

Omitting numerous fabrication details which appear in the text anddrawings of the '750 patent, it discloses in a first example thatmaterial forming its beam initially begins as part of a monolithicp-type silicon substrate which carries an n-type diffusion layer intowhich boron ions are injected to form a p⁺ surface layer. That is, then-type diffusion layer separates the p⁺ surface layer from the p-typesilicon substrate. During the beam's fabrication, etching removes thep-type silicon substrate leaving only material of the n-type diffusionlayer and p⁺ surface layer to form the beam. Similarly, torsion barfabrication removes material of the n-type diffusion layer leaving onlymaterial of p⁺ surface layer to form the torsion bars. Subsequentprocessing forms aluminum support members spanning between the p⁺surface layer material forming the torsion bar ends and the adjacentglass substrate.

The '540 patent discloses that to reduce switch insertion loss as wellas improve sensitivity, its beam is preferably formed from entirely ofmetal as is the pin about which the beam rotates. In particular, the'540 patent discloses that the beam may be formed from nickel (“Ni”)electroplated at low temperatures compared to most semiconductorprocessing. The '540 patent discloses that not only does its all metalbeam reduce insertion losses relative to known SiO₂ or composite siliconmetal beams, such a configuration also improves the third orderintercept point for providing increased dynamic range. Electricalpotentials applied respectively between a pair of gold electrodesdeposited on one side of the glass substrate nearest to the metallicbeam and a pair of field plates disposed on the opposite side of theglass substrate furthest from the beam generate the electrostatic forcewhich effects rotation of the beam about the metallic pin.

The vane included in the MEMS switch disclosed in the '091 patent isformed of relatively inflexible material, such as plated metal,evaporated metal, or dielectric material on top of a metal seed layer.Thin flexible metal hinges connect opposite sides of the vane to a goldframe which projects outward from the low-loss microwave insulating orsemi-insulating substrate. The substrate may be fabricated from quartz,alumina, sapphire, Low Temperature Ceramic Circuit on Metal (“LTCC-M”),GaAs or high-resistivity silicon. Configured in this way, the vane andthe hinges are disposed above the substrate, and the flexible hingeselectrically couple the vane to the frame. The hinges, which can be flator corrugated, allow the vane to rotate about a pivot axis that isparallel to the substrate and above the lower fulcrum. Pull-back andpull-down electrodes, which can be encapsulated with an insulator suchas silicon nitride (Si₃N₄), are formed on the substrate adjacent to thevane. Electrical potentials applied either to the pull-down or thepull-back electrodes respectively close or open the MEMS switch.

A series of U.S. Pat. Nos. 5,629,790, 5,648,618, 5,895,866, 5,969,465,6,044,705, 6,272,907, 6,392,220 and 6,426,013 all disclose MEMSstructured which are reminiscent to a greater or lesser extent to thosedescribed above for the '750, '540 and '091 patents. These patents alldisclose an integrated, micromachined torsional scanner, which in aparticular configuration, may include a frame-shaped reference member. Aparticular configuration of the torsional scanner includes a pair ofdiametrically opposed, axially aligned torsion bars that are coupled toand project from the reference member. In a particular configuration, aplate-shaped dynamic member, analogous to the beams and vane disclosedrespectively in the '750, '540 and '091 patents, is encircled by theframe and is coupled thereto by the torsion bars. Configured in thisway, the torsion bars support the dynamic member for rotation about anaxis that is collinear with the torsion bars. The reference member, thetorsion bars and the dynamic member are all monolithically fabricatedfrom a semiconductor layer of a silicon substrate. A desirable methodfor fabricating the torsional scanner uses a Simox wafer, or similarwafers, e.g. a silicon-on-insulator (“SOI”) substrate, where thethickness of the plate is determined by an epitaxial layer of the wafer.As compared to metals or polysilicon, single crystal silicon ispreferred both for the plate and for the torsion bars because of itssuperior strength and fatigue characteristics. These patents alsodisclose using electrostatic force to effect rotary motion of thedynamic member.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an improved MEMSswitch.

Another object of the present invention is to provide a MEMS switch thatswitches swiftly.

Another object of the present invention is to provide a MEMS switchhaving a lower operating voltage.

Another object of the present invention is to provide a single-poledouble-throw (“SPDT”) MEMS switch.

Another object of the present invention is to provide a MEMS switchwhich by routine structural repetition can provide additional poles.

Another object of the present invention is to provide a MEMS switch thatprovides improved signal isolation.

Another object of the present invention is to provide a MEMS switchwhich facilitates switch contact material selection and customization.

Another object of the present invention is to provide a MEMS switchwhose manufacture does not require a sacrificial layer.

Another object of the present invention is to provide a MEMS switch thatfacilitates bulk manufacture, and divides facilely into individual MEMSswitches.

Another object of the present invention is to provide a MEMS switch thatinherently becomes hermetically sealed during fabrication.

Another object of the present invention is to provide a MEMS switchwhich is simpler.

Another object of the present invention is to provide a MEMS switch thatis cost effective.

Another object of the present invention is to provide a MEMS switch thatis easy to manufacture.

Another object of the present invention is to provide a MEMS switch thatis economical to manufacture.

Another object of the present invention is to provide a MEMS structurewhich provides a good electrical connection between metal present on twodifferent layers of the MEMS structure.

Briefly, a first aspect of the present invention is an integral MEMSswitch that is adapted for selectively coupling an electrical signalpresent on a first input conductor connected to the MEMS switch to afirst output conductor also connected to the MEMS switch. The MEMSswitch includes a micro-machined monolithic layer of material having:

-   -   a. a seesaw;    -   b. a pair of torsion bars that are disposed on opposite sides of        and coupled to the seesaw, and which establish an axis about        which the seesaw is rotatable; and    -   c. a frame to which ends of the torsion bars furthest from the        seesaw are coupled.        The frame supports the seesaw through the torsion bars for        rotation about the axis established by the torsion bars. The        MEMS switch also includes an electrically conductive shorting        bar carried at an end of the seesaw that is located away from        the rotation axis established by the torsion bars.

The MEMS switch also includes a base that is joined to a first surfaceof the monolithic layer. A substrate, also included in the MEMS switch,is bonded to a second surface of the monolithic layer that is locatedaway from the first surface thereof to which the base is joined. Formedin the substrate are an electrode which is juxtaposed with a surface ofthe seesaw that is located to one side of the rotation axis establishedby the torsion bars. Upon application of an electrical potential betweenthe electrode and the seesaw, the seesaw is urged to rotate in a firstdirection about the rotation axis established by the torsion bars. Alsoformed on the substrate are a pair of switch contacts that are adaptedto be connected respectively to the input conductor and to the outputconductor. The pair of switch contacts:

-   -   a. are disposed adjacent to but spaced apart from the first        shorting bar when no force is applied to the seesaw;    -   b. are electrically insulated from each other when no force is        applied to the seesaw; and    -   c. upon application of a sufficiently strong force to the seesaw        which urges the seesaw to rotate in the first direction, are        contacted by the first shorting bar.        In this way, contact between the shorting bar and the switch        contacts electrically couples together the first pair of switch        contacts.

Another aspect of the present invention is a MEMS electrical contactstructure and a MEMS structure which includes a first and a second layereach of which respectively carries an electrical conductor. The secondlayer also includes a cantilever which supports an electrical contactisland at a free end of the cantilever. The electrical contact islandhas an end which is distal from the cantilever, and which carries aportion of the electrical conductor that is disposed on the secondlayer. In this particular aspect of the present invention the portion ofthe electrical conductor at the end of the electrical contact island isurged by force supplied by the cantilever into intimate contact with theelectrical conductor that is disposed on the first layer.

These and other features, objects and advantages will be understood orapparent to those of ordinary skill in the art from the followingdetailed description of the preferred embodiment as illustrated in thevarious drawing figures.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a seesaw, electrodes, switch contacts,and shorting bars that are included in MEMS switches in accordance withthe present invention;

FIGS. 2A and 2B are alternative elevational views of the seesaw,electrodes, electrodes, switch contacts, and shorting bars taken alongthe line 2A,2B—2A,2B in FIG. 1;

FIG. 3 is a perspective view of an area on a surface of a base waferincluded in the MEMS switch into which micro-machined cavities have beenformed in accordance with a preferred embodiment of the presentinvention;

FIG. 4 is a perspective view illustrating fusion bonding of a devicelayer of an SOI wafer onto a top surface of the base wafer into whichcavities have been micro-machined;

FIG. 5 is a perspective view of the device layer of the SOI wafer fusionbonded onto the top surface of the base wafer after removal of the SOIwafer's handle layer and buried SiO₂ layer;

FIG. 6 is a perspective view of a portion of the device layer of the SOIwafer fusion bonded onto the top surface of the base wafer that islocated immediately over the area of the base wafer depicted in FIG. 3after formation of an initial cavity therein and deposition andpatterning of an electrically insulating SiO₂ layer;

FIG. 7 is another perspective view of a portion of the device layer ofthe SOI wafer fusion bonded onto the top surface of the base waferillustrated in FIG. 6 after deposition of metallic structures in theinitial cavity and formation of the seesaw and its supporting torsionbars;

FIG. 8 is a plan view of the central portion of the initial cavity takenalong the line 8—8 in FIG. 7 showing the metallic structures, the seesawand its supporting torsion bars which are located there;

FIG. 9 is a perspective view of a portion of a glass substrate to bemated with the area of the device layer depicted in FIG. 7 whichillustrates metal structures micro-machined thereon;

FIG. 10 is a perspective view of portions of the base wafer, the devicelayer of the SOI wafer, and the glass substrate depicted in FIG. 9 afterthe metallic structures on the glass substrate have been mated with themicro-machined surface of the device layer depicted in FIG. 7, and thedevice layer has been anodically bonded thereto;

FIG. 11 is a perspective view of a portion of the basic wafer, devicelayer and glass substrate depicted in FIG. 10 after the basic wafer andglass substrate have been thinned, and after micro-machining aperturesthrough the basic wafer there by exposing contact pads and groundingpads that are included among the micro-machined metallic structuresdepicted in FIG. 7;

FIG. 12 is a cross-sectional, elevational view taken along the line12—12 in FIG. 11 illustrating wire bonding an electrical lead to one ofthe several contact pads included in the MEMS switch;

FIG. 13 is a perspective view of a portion of the basic wafer, devicelayer and glass substrate depicted in FIGS. 10 and 11 after the basicwafer and glass substrate have been thinned, and after sawing the basicwafer there by exposing contact pads and grounding pads that areincluded among the micro-machined metallic structures depicted in FIG.7;

FIG. 14 is a cross-sectional, elevational view taken along the line14—14 in FIG. 13 illustrating wire bonding an electrical lead to one ofthe several contact pads included in the MEMS switch;

FIG. 15 is a perspective view of a portion of the basic wafer, devicelayer and glass substrate depicted in FIG. 10 after the basic wafer andglass substrate have been thinned for another alternative embodiment ofthe present invention in which electrically conductive vias are formedthrough the glass substrate;

FIG. 16 is a cross-sectional, elevational view taken along the line16—16 in FIG. 15 illustrating several vias formed through the glasssubstrate that effect an electrical connection to contact and groundingpads included in the MEMS switch;

FIG. 17 is a perspective view of a portion of an alternative embodimentglass substrate which illustrates micro-machined channels which holdelectrical conductors;

FIG. 18 is a perspective view of a portion of the alternative embodimentglass substrate depicted in FIG. 17 with the channels and electricalconductors juxtaposed with a support wafer to which the glass substratehas been anodically bonded to permit forming electrically conductivevias through the glass substrate;

FIG. 19 is a perspective view of portions of the base wafer and thedevice layer of the SOI wafer similar to that depicted in FIG. 7 and theglass substrate and support wafer depicted in FIG. 18 after the metallicstructures, including electrically conductive vias, have been mated withthe micro-machined surface of the device layer, and the device layer hasbeen anodically bonded to the glass substrate; and

FIG. 20 is a cross-sectional, elevational view taken along the line20—20 in FIG. 19 illustrating several vias formed through the glasssubstrate that effect an electrical connection to bonding pads includedin the MEMS switch.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1, 2A and 2B illustrate a seesaw 52, metallic electrodes 54 a and54 b, metallic switch contacts 56 a 1, 56 a 2, 56 b 1 and 56 b 2, andmetallic shorting bars 58 a and 58 b that are included in MEMS switchesof the present invention. The seesaw 52 is formed by micro-machining alayer 62 of material, preferably single crystal silicon (Si). Materialof the layer 62 also forms a frame 64 which preferably surrounds theseesaw 52. A pair of torsion bars 66 a and 66 b, which are depicted bydashed lines in FIG. 1 and which extend outward from opposite sides ofthe seesaw 52 to the frame 64, are also formed monolithically with theseesaw 52 and the frame 64 from the material of the layer 62. Whiledimensions of the seesaw 52 vary depending upon a particularconfiguration for the MEMS switch, in one illustrative embodiment theaperture micro-machined into the layer 62 to establish the frame 64which surrounds the seesaw 52 measures approximately about 0.4×0.4millimeters. In this same illustrative embodiment, the layer 62 isapproximately 17 microns thick, while the seesaw 52 is approximately 5microns thick as are the torsion bars 66 a and 66 b.

The torsion bars 66 a and 66 b support the seesaw 52 from thesurrounding frame 64 for rotation about an axis 68 which is collinearwith the torsion bars 66 a and 66 b. The shorting bars 58 a and 58 b,which are several microns thick, are carried by the seesaw 52 atopposite ends thereof which are furthest from the axis 68. The torsionbars 66 a and 66 b are approximately 20 microns wide and 60 microns longin the previously mentioned illustrative embodiment. The torsion bars 66a and 66 b having this configuration are stiff and therefore exhibit ahigh resonant frequency, and provide a very large restoring force whichreduces the likelihood that MEMS switches will exhibit stiction.Furthermore, stiffness of the torsion bars 66 a and 66 b is directlyrelated to switching speed with a higher the resonant frequency for thecombined seesaw 52 and torsion bars 66 a and 66 b increasing theswitching speed.

For the illustrative embodiment described above, several microns of gold(Au) plated onto a thin titanium (Ti) adhesion layer forms the shortingbars 58 a and 58 b. The shorting bars 58 a and 58 b are approximately 10microns wide, and 40 microns long. A pair of silicon dioxide (SiO₂)insulating pads 72 a and 72 b, respectively located at opposite ends ofthe seesaw 52 furthest from the axis 68, are interposed between theshorting bars 58 a and 58 b and the seesaw 52 to electrically insulatethe shorting bars 58 a and 58 b therefrom. As depicted in FIG. 1, the 72b{tilde over ( )}insulating pads 72 a and 72 b cover a larger area onthe seesaw 52 than the shorting bars 58 a and 58 b and are approximately1.0 micron thick. The electrodes 54 a and 54 b and the switch contacts56 a 1, 56 a 2, 56 b 1 and 56 b 2 adjacent to the seesaw 52 areapproximately 4.0 microns thick.

When there is no external force applied to the seesaw 52, the restoringforce supplied by the torsion bars 66 a and 66 b disposes the seesaw 52in the position illustrated in FIG. 2A. Disposed in this position, adistance of approximately 3 microns separates the seesaw 52 from theadjacent electrodes 54 a and 54 b and switch contacts 56 a 1, 56 a 2, 56b 1 and 56 b 2. Applying an electrical potential between the layer 62and one of the electrodes 54 a and 54 b causes the seesaw 52 to rotateabout the axis 68 due to the attraction of the seesaw 52 toward thatelectrode, e.g. electrode 54 a in FIG. 2B. Sufficient rotation of theseesaw 52 causes one of the shorting bars 58 a and 58 b to contact apair of the switch contacts 56 a 1 and 56 a 2, or 56 b 1 and 56 b 2,e.g. switch contacts 56 a 1 and 56 a 2 in FIG. 2B, to establish anelectrical circuit there between.

While as described below there exist various different processes forassembling a MEMS switch in accordance with the present invention havingthe seesaw 52, electrodes 54 a and 54 b, switch contacts 56 a 1, 56 a 2,56 b 1 and 56 b 2, and shorting bars 58 a and 58 b configured asillustrated in FIGS. 1, 2A and 2B, a preferred process begins asdepicted in FIG. 3. FIG. 3 depicts an area 102 occupied by a single MEMSswitch on a base wafer 104. In the illustration of FIG. 3, lines 106indicate boundaries of the central area 102 with eight (8) identical,adjacent areas 102 which, except adjacent to edges of the base wafer104, surround the central area 102. In accordance with the followingdescription, after the MEMS switch has been completely fabricated, theareas 102 will be separated into those of individual MEMS switches bysawing along the lines 106.

The base wafer 104 is a conventional silicon wafer which may be thinnerthan a standard SEMI thickness for its diameter. For example, if thebase wafer 104 has a diameter of 150 mm, then a standard SEMI waferusually has a thickness of approximately 650 microns. However, thethickness of the base wafer 104, which can vary greatly and still beusable for fabricating a MEMS switch in accordance with the presentinvention, may be thinner than a standard SEMI silicon wafer.

Fabrication of the preferred embodiment of a MEMS switch in accordancewith the present invention begins first with micro-machining aswitched-terminals pad cavity 112, a seesaw cavity 114 and acommon-terminal pad cavity 116 into a top surface 108 of the base wafer104. The depth of the cavities 112, 114 and 116 is not critical, butshould be approximately 10 microns deep for the illustrative embodimentdescribed above. A plasma system, preferably a Reactive Ion Etch (“RIE”)that will provide good uniformity and anisotropy, is used inmicro-machining the cavities 112, 114 and 116. However, KOH or other wetetches may also be used to micro-machine the cavities 112, 114 and 116.A standard etch blocking technique is used in micro-machining thecavities 112, 114 and 116, i.e. either photo-resist for plasma etchingor a mask formed either by silicon oxide or silicon nitride for a wet,KOH etch. This micro-machining produces the seesaw cavity 114 whichaccommodates movement of the seesaw 52 such as that illustrated in FIG.2B, while the cavities 112 and 116 as described in greater detail belowaccommodate feedthroughs or electrical contact pads.

After the cavities 112, 114 and 116 have been micro-machined into thetop surface 108, the next step, not illustrated in any of the FIGS., isetching alignment marks into a bottom surface 118 of the base wafer 104depicted in FIG. 3. The bottom side alignment marks must register withthe cavities 112, 114 and 116 micro-machined into the base wafer 104 topermit aligning other structures micro-machined during subsequentprocessing operations with the cavities 112, 114 and 116. These bottomside alignment marks will also be used during a bottom side silicon etchnear the end of the entire process flow. The bottom side alignment marksare established first by a lithography step using a specialtarget-only-mask, aligned with the cavities 112, 114 and 116, and thenby micro-machining the bottom surface 118 of the base wafer 104. Thepattern of the target-only-mask is plasma etched a few microns deep intothe bottom surface 118 before removing photo-resist from both surfacesof the base wafer 104. Creating bottom side alignment marks can beomitted if an aligner having infrared capabilities is available for usein fabricating MEMS switches.

The next step in fabricating the MEMS switch, depicted in FIG. 4, isfusion bonding a thin, single crystal Si device layer 122 of asilicon-on-insulator (“SOI”) wafer 124 to the top surface 108 of thebase wafer 104. Preferably the device layer 122 of the SOI wafer 124 is17 microns thick over an extremely thin buried layer of silicon dioxide(SiO₂), thus its name Silicon on Insulator or SOI. A characteristic ofthe SOI wafer 124 which is advantageous in micro-machining the seesaw 52and the torsion bars 66 a and 66 b is that the device layer 122 has anessentially uniform thickness, preferably about 17 microns, over theentire surface of the SOI wafer 124 with respect to the thin SiO₂ layer132. In fusion bonding the device layer 122 of the SOI wafer 124 to thetop surface 108 of the base wafer 104, the wafers 104 and 124 arealigned globally by matching an alignment flat 134 on the base wafer 104with a corresponding alignment flat 136 on the SOI wafer 124. Fusionbonding of the SOI wafer 124 to the base wafer 104 is performed atapproximately 1000° C.

After the base wafer 104 and the SOI wafer 124 have been formed into asingle piece by fusion bonding, a handle layer 138 located furthest fromthe device layer 122 and then the SiO₂ layer 132 are removed leavingonly the device layer 122 bonded to the top surface 108 of the basewafer 104. First a protective silicon dioxide layer, a silicon nitridelayer, a combination of both, or any other suitable protective layer isformed on the bottom surface 118 of the base wafer 104. Having thusmasked the base wafer 104, the silicon of the handle layer 138 isremoved using a KOH etch applied to the SOI wafer 124. Upon reaching theburied SiO₂ layer 132 after the bulk of the silicon forming the handlelayer 138 has been removed, the rate at which the KOH etches the SOIwafer 124 slows appreciably. In this way, the SiO₂ layer 132 functionsas an etch stop for removing the handle layer 138. After the bulksilicon of the handle layer 138 has been removed, the formerly buriedbut now exposed SiO₂ layer 132 is removed using a HF etch. Note thatother methods of removing the bulk silicon of the handle layer 138 maybe used including other wet silicon etchants, a plasma etch, grindingand polishing, or a combination of methods. After completing thisprocess only the device layer 122 of the SOI wafer 124 remains bonded tothe base wafer 104 as illustrated in FIG. 5.

FIG. 6 depicts what has been exposed as a front surface 142 of devicelayer 122 due to etching away of the handle layer 138 and the SiO₂ layer132. Similar to forming the cavities 112, 114 and 116, the next step infabricating the preferred embodiment of the MEMS switch ismicro-machining, preferably using a KOH etch, an approximately 12.0micron deep initial cavity 144 through the front surface 142 into thedevice layer 122. As is well known to those skilled in the art of MEMSand semiconductor fabrication, the front surface 142 of the device layer122 is first oxidized and patterned to provide a blocking mask formicro-machining the initial cavity 144 using KOH. The oxide on the frontsurface 142 of the device layer 122 remaining after micro-machining theinitial cavity 144 is then removed. While the illustration of FIG. 6 etseq. depict the walls of the initial cavity 144 as being vertical,because they are preferably formed using a KOH etch rather than a RIEplasma etch, as is well known in the art the walls of the initial cavity144 in the preferred embodiment actually slope at an angle ofapproximately 54°.

In the preferred embodiment of the MEMS switch, the depth of the initialcavity 144 establishes a spacing between surfaces of the electrodes 54 aand 54 b, illustrated in FIG. 2A, that are furthest from the seesaw 52,and a surface of the seesaw 52 nearest to the electrodes 54 a and 54 b.The depth of the initial cavity 144 is calculated to provide the desiredgap between the shorting bars 58 a and 58 b on the seesaw 52 and themetal of the electrodes 54 a and 54 b and the switch contacts 56 a 1, 56a 2, 56 b 1 and 56 b 2 taking into consideration the desired thicknessof the seesaw 52 and of the thin device layer 122.

Micro-machining the initial cavity 144 into the device layer 122 leavesfour (4) grounding islands 152 projecting upward from a floor of theinitial cavity 144, a U-Shaped wall 154 and also a serrated U-shapedwall 156. The grounding islands 152 and the walls 154 and 156 extendupward from a floor of the initial cavity 144 to the front surface 142of the device layer 122. The walls 154 and 156 mainly surround an areaof the floor of the front surface 142 which is to become the seesaw 52of the MEMS switch. After forming the initial cavity 144, the SiO₂insulating pads 72 a and 72 b are deposited onto the floor of theinitial cavity 144 in preparation for depositing the shorting bars 58 aand 58 b and other metallic structures within the initial cavity 144.

FIGS. 7 and 8 depict various metallic structures, including the shortingbars 58 a and 58 b, which are deposited on the floor of the initialcavity 144. As stated previously, these metallic structures arepreferably formed by first depositing a thin Ti adhesion layer ontowhich is then deposited, the illustrative embodiment, approximately 0.5microns of Au. In addition to the shorting bars 58 a and 58 b, a pair ofmetallic ground plates 162 a and 162 b respectively extend across theinitial cavity 144 past the shorting bars 58 a and 58 b and insulatingpads 72 a and 72 b between pairs of grounding islands 152. Afterdepositing the 0.5 micron Au layer, the metal is then lithographicallypatterned and etched to establish shapes for the shorting bars 58 a and58 b and the ground plates 162 a and 162 b. Subsequently, additional Auis plated onto the shorting bars 58 a and 58 b for a total thickness ofapproximately 4.0 microns.

After all the metallic structures have been formed in the initial cavity144, a second RIE etch, which pierces material of the device layer 122remaining at the floor of the initial cavity 144, outlines the torsionbars 66 a and 66 b and the seesaw 52 thereby freeing the seesaw 52 forrotation about the axis 68. In this way the seesaw 52 and torsion bars66 a and 66 b are formed monolithically with the surrounding material ofthe device layer 122 which becomes the frame 64. The second RIE etchalso opens the initial cavity 144 to the cavities 112 and 116 in thebase wafer 104 leaving cantilevers 166 beneath and supporting each ofthe grounding islands 152. Supporting each grounding island 152 at afree end of a cantilever 166 accommodates the thickness of the Au at theends of the ground plates 162 a and 162 b atop each grounding island 152which projects above the front surface 142. Compliant force supplied bythe cantilever 166 ensures formation of a good electrical contactbetween the ground plates 162 a and 162 b and subsequent metalizationlayers described below.

FIG. 9 depicts an area on a metalization surface 172 of a Pyrex glasssubstrate 174 which subsequently will be mated with and fused to thefront surface 142 of the device layer 122 depicted in FIG. 7. The glasssubstrate 174 has the same diameter as the base wafer 104 and SOI wafer124, and preferably is 1.0 mm thick. The illustration of FIG. 9 depictsmetal structures present atop the metalization surface 172 afterdepositing a thin 1000 A° seed layer of chrome-gold (Cr—Au) onto themetalization surface 172. Patterning of the Cr—Au seed layer establishescontact pads and conductor lines for what will become a common terminal182 of the preferred embodiment MEMS switch, the switch contacts 56 a 1,56 a 2, 56 b 1 and 56 b 2, and the electrodes 54 a and 54 b. Patterningof the Cr—Au seed layer also establishes grounding pads 186 that areadapted for mating with and engaging that portion of the ground plates162 a and 162 b which is present on projecting ends of the groundingislands 152. After patterns have been established in the Cr—Au seedlayer for these structures, approximately 2.0 microns of Au is thenplated to form the patterns which appear in FIG. 9. Preferably theswitch contacts 56 a 1, 56 a 2, 56 b 1 and 56 b 2 and the commonterminal 182 are 4.0 micron thick to satisfy skin effect requirementsassociated with efficiently conducting high frequency radio frequency(“RF”) signals. However, a switch in accordance with the presentinvention may use materials and processing procedures which differ fromthose described above.

The electrodes 54 a and 54 b are plated to the same thickness as theswitch contacts 56 a 1, 56 a 2, 56 b 1 and 56 b 2 to reduce the gapbetween the electrodes 54 a and 54 b and immediately adjacent areas onthe seesaw 52. A smaller gap between the electrodes 54 a and 54 b andimmediately adjacent areas on the seesaw 52 reduces voltage which mustbe applied to actuate the MEMS switch.

FIG. 10 depicts the area of the base wafer 104, illustratedprogressively in FIGS. 3, 6 and 7, after the corresponding area of themetalization surface 172 of the glass substrate 174, illustrated in FIG.9, has been anodically bonded to the front surface 142 of the devicelayer 122. In bonding the metalization surface 172 to the front surface142, the metal pattern depicted in FIG. 9 is carefully aligned with thestructure micro-machined into the device layer 122 that appears in FIGS.7 and 8. Bonding of the metalization surface 172 to the front surface142 in this way establishes the MEMS switch as illustrated in FIGS. 1,2A and 2B. In the structure depicted in FIGS. 7 and 8, the wires of theelectrodes 54 a and 54 b connecting to the contact pads thereofrespectively pass through the serrations in the wall 156 while theswitch contacts 56 a 1, 56 a 2, 56 b 1 and 56 b 2 respectively passalong arms of the U-shaped walls 154 and 156 in close proximityrespectively to the ground plates 162 a and 162 b.

During anodic bonding of the metalization surface 172 to the 174, thecantilevers 166 supporting the grounding islands 152 deflect due tointerference between the metal of the ground plates 162 a and 162 b thatis atop each grounding island 152 and of the grounding pads 186 formedon the metalization surface 172 of the glass substrate 174. Mechanicalstiffness of the single crystal silicon material forming the cantilevers166 provides forces which ensure a sound electrical connection betweenthe grounding pads 186 and the portions of the ground plates 162 a and162 b juxtaposed therewith at the grounding islands 152.

After the glass substrate 174 has been anodically bonded to the wall154, the entire outer portions both of the base wafer 104 and of theglass substrate 174 furthest from the device layer 122 are thinned asindicated by dashed lines 192 and 194 in FIG. 10. Preferably, the basewafer 104 and of the glass substrate 174 are thinned in a double sidegrinding and polishing operation. About half the thickness of each layeris removed with the glass substrate 174 having a final thickness ofapproximately 100 microns. Grinding and polishing of the combined basewafer 104, device layer 122 and glass substrate 174 yields MEMS switcheshaving a thickness comparable to that of standard semiconductor devices.Any techniques commonly used in MEMs or semiconductor processing,including grinding, polishing, chemical mechanical planarization(“CMP”), or various wet or plasma etches, may be used in thinning thebase wafer 104 and the glass substrate 174.

FIG. 11 depicts the section of the combined base wafer 104, device layer122 and glass substrate 174 inverted from the illustration of FIG. 10.FIG. 11 also illustrate apertures etched through silicon material of thebase wafer 104 which before etching remained at the base of the cavities112 and 116 after thinning the base wafer 104. Extending the cavities112 and 116 is performed by first establishing a pattern on the bottomside of the base wafer 104 furthest from the device layer 122 using adouble-side aligner and viewing the structure of the device layer 122through the transparent glass substrate 174. Then the silicon materialforming the base wafer 104 is plasma etched using a deep RIE system.Opening the cavities 112 and 116 in this way exposes the contact padsfor the electrodes 54 a and 54 b, the switch contacts 56 a 1 and 56 b 1together with the common terminal 182 for switch contacts 56 a 2 and 56b 2, and the grounding pads 186, depicted in FIG. 9 and by dashed linesin FIG. 11, that were initially formed on the glass substrate 174 priorto anodic bonding.

FIG. 12 is a cross-sectional view of a MEMS switch in accordance withthe present invention after sawing of the combined base wafer 104,device layer 122 and glass substrate 174 to individualize the manyswitches concurrently fabricated therein, and after wire bondingelectrical leads 198 to contact pads and grounding pads 186 included inthe MEMS switch, only one of which electrical leads 198 appears in FIG.12.

The electrical leads 198 provides a means for coupling two input signalsinto the MEMS switch one of which is output therefrom, or alternativelycoupling a single input signal to either one or the other of two outputsfrom the MEMS switch. The electrical leads 198 also provides means forelectrically grounding the ground plates 162 a and 162 b together withthe seesaw 52, and for establishing a difference in electrical potentialbetween the seesaw 52 and the electrodes 54 a and 54 b which urge theseesaw 52 to rotate about the axis 68.

Sawing the combined base wafer 104, device layer 122 and glass substrate174 produces individual MEMS switches which typically are approximately2.0×1.5×1.5 millimeters (L×W×H). These dimensions can easily vary to betwice as large or one-half that size. During sawing of the combined basewafer 104, device layer 122 and glass substrate 174, open cavities 112and 116 on the surface of the base wafer 104 which face upward arecovered by conventional wafer tape. Sealing the cavities 112 and 116with the wafer tape is important to insure the saw slurry does not enterinto the cavities 112 and 116 where contact pads and grounding pads 186are exposed at bases thereof, and, perhaps, even to the shorting bars 58a and 58 b and switch contacts 56 a 1, 56 a 2, 56 b 1 and 56 b 2 at theinterior of the MEMS switch.

If necessary or advantageous, a barrier to intrusion of the saw slurryinto the interior of the MEMS switch may also be established by makingsurfaces of the device layer 122 depicted in FIG. 7 and the glasssubstrate 174 depicted in FIG. 9 hydrophobic. Passages between thecavities 112 and 116 and the interior of the MEMS switch where theshorting bars 58 a and 58 b and switch contacts 56 a 1, 56 a 2, 56 b 1and 56 b 2 established during anodic bonding of the glass substrate 174to the device layer 122 are approximately 10 microns by 100 microns. Ifsurfaces of these passages are hydrophobic, that surface condition willbar intrusion of water during sawing. Making these surfaces hydrophobicis accomplished by coating the surfaces with silicone before anodicallybonding the metalization surface 172 of the glass substrate 174 thereto,or after etching the backside of the base wafer 104 as described aboveto open the cavities 112 and 116. One method that maybe used for coatingthe surfaces with silicone involves placing the combined base wafer 104and device layer 122 depicted in FIG. 7 or the combined base wafer 104,device layer 122 and glass substrate 174 depicted in FIG. 11 into avacuum chamber with a heated pad of Gel Pak material. A hot plate isused to heat a layer of polymer from the Gel Pak pad to approximately40° C. After the hot plate has reached this temperature, the chambercontaining the combined base wafer 104 and device layer 122 and the GelPak pad is sealed, evacuated and left in that state for approximately 4hours. After that interval of time, the chamber is first purged thenbackfilled with air and then the combined base wafer 104 and devicelayer 122 removed for subsequent processing. Processing the combinedbase wafer 104 and device layer 122 in this way prevents water fromentering the interior of the MEMS switch through the cavities 112 and116 during sawing.

Alternative embodiments of the present invention mainly involvedifferent techniques for making electrical connections to the switchcontacts 56 a 1, 56 a 2, 56 b 1 and 56 b 2, electrodes 54 a and 54 b,and ground plates 162 a and 162 b. One alternative technique forproviding these connections illustrated in FIGS. 13 and 14 machines sawcuts 204 along rows of cavities 112 and 116 into but not through thebase wafer 104, rather than RIE etching, for opening the cavities 112and 116. Depending upon the spacing between immediately adjacent MEMSswitches in the combined base wafer 104, device layer 122 and glasssubstrate 174 and upon the width of the saw blade, machining the sawcuts 204 may, or may not, leave a projecting ridge 206 betweenimmediately adjacent pairs of saw cuts 204. Subsequent sawing completelythrough the combined base wafer 104, device layer 122 and glasssubstrate 174 to form individual MEMS switches removes the ridge 206, ifone remains. Because machining the saw cuts 204 necessarily exposes thecontact and grounding pads to saw slurry, for this particularalternative embodiment it is essential that the passages between thecavities 112 and 116 and the interior of the MEMS switch be madehydrophobic before anodically bonding the glass substrate 174 to thedevice layer 122. Preferably these surfaces are rendered hydrophobicusing the Gel Pak procedure described above.

Another alternative technique for providing the required electricalconnections follows, with two main differences, the same procedure forfabricating the MEMS switch as that set forth above through thinning thebase wafer 104 and the glass substrate 174 depicted in FIG. 10. Thefirst difference is that the cavities 112 and 116 depicted in FIG. 3 arenot required for electrical contact pads, but are only necessary for thegrounding islands 152 and the cantilevers 166. In this alternativeembodiment the contact and grounding pads will be located on the outerlayer of the glass substrate 174. The second difference is that themetal pattern will differ form the preferred embodiment to optimize RFperformance utilizing two layers of metal interconnects, on each side ofthe glass wafer. After thinning the glass substrate 174 to a thicknessof approximately 50 microns, as depicted in FIGS. 15 and 16 vias 212 areetched through the glass substrate 174 to the Cr seed layer of contactpads, grounding pads and electrodes. The Cr seed layer was deposited informing the metal structures depicted in FIG. 9. The glass is typicallywet etched using an isotropic etchant such as 8:1 HNO₃:HF. The etchantwill stop on reaching the Cr layer. After the metal forming the contactpads, grounding pads and electrodes has been exposed, metal 214 isdeposited into the vias 212 and over the surface of the glass substrate174 thereby extending the metal of the contact pads, grounding pads andelectrodes to the outer surface of the glass substrate 174. The metal214 is a sputtered or evaporated film of chrome-gold (Cr—Au) similar tothat deposited on the glass substrate 174 in forming the metalstructures depicted in FIG. 9. The deposited Cr—Au film is patterned andetched leaving bonding pad areas adjacent and connected to the metal 214deposited into each of the. Subsequently, additional Au is plated on themetal for a total thickness of approximately 4.0 microns. The bondingpad areas of the metal 214 may then be connected to a printed circuitboard either by wires bonded to the metal 214 or by solder bumps. RIEetching of the base wafer 104 to open cavities 112 and 116 asillustrated in FIG. 11 is no longer necessary since the bonding padareas are provided on the external surface of the glass substrate 174.Therefore the backside patterning and etching of the base wafer 104needed for RIE etching to open the cavities 112 and 116 is omitted inthis alternative embodiment. One advantage provided by this particularalternative technique for forming electrical connections to the switchcontacts 56 a 1, 56 a 2, 56 b 1 and 56 b 2, electrodes 54 a and 54 b,and ground plates 162 a and 162 b is that the resulting MEMS switch ishermetically sealed.

FIGS. 17 through 20 depict a final alternative embodiment which alsoproduces a hermetically sealed MEMS switch. In this alternativeembodiment, first a pattern of channels 222 are etched approximately 50microns deep into a surface 224 of the glass substrate 174 as depictedin FIG. 17. A seed layer of Cr—Au is then deposited onto the surface 224and patterned to permit subsequently forming Au conductors 226 in eachof the channels 222 which are approximately 4.0 microns thick. The Auconductors 226 carry the electrical signals from the switch structures,i.e. the switch contacts 56 a 1, 56 a 2, 56 b 1 and 56 b 2, electrodes54 a and 54 b and ground plates 162 a and 162 b, within the hermeticallysealed part of the MEMS switch to bonding pads 248 that are outside thesealed portion of the MEMS switch.

As depicted in FIG. 18, the surface 224 of the glass substrate 174 isthen anodically bonded to a conventional silicon support wafer 232, andthe glass substrate 174 thinned to 100 microns. Similar to the processdescribed above for the alternative embodiment depicted in FIGS. 15 and16, vias 242 are then etched through the glass substrate 174 to the Crseed layer of the conductors 226. The glass is typically wet etchedusing an isotropic etchant such as 8:1 HNO₃:HF. The etchant will stop onreaching the Cr layer. After the Cr layer of the conductors 226 has beenexposed, metal 244 is deposited into the vias 242 and over themetalization surface 172 of the glass substrate 174 thereby extendingthe metal of the conductors 226 to the metalization surface 172 of theglass substrate 174. The metal 244 is a sputtered or evaporated film ofchrome-gold (Cr—Au) similar to that deposited on the glass substrate 174in forming the metal structures depicted in FIG. 9. The deposited Cr—Aufilm is patterned and etched to form the electrodes 54 a and 54 b, theswitch contacts 56 a 1, 56 a 2, 56 b 1 and 56 b 2, contacts for theground plates 162 a and 162 b atop the grounding islands 152 as well asbonding pads 248. Subsequently, additional Au is plated on the metal fora total thickness of approximately 4.0 microns.

The metalization surface 172 of the glass substrate 174 is thenanodically bonded to the front surface 142 of the device layer 122 asillustrated in FIG. 19 so the bonding pads 248 become isolated from theremainder of the MEMS switch in bonding pad cavities 252. The cavities252, which are located immediately adjacent to where saw cuts willsubsequently individualize the MEMs switches, are formed into the basewafer 104 concurrently with micro-machining the cavities 112, 114 and116 depicted in FIG. 6, and through the device layer 122 concurrentlywith micro-machining the initial cavity 144 in FIG. 6 and then freeingthe seesaw 52 in FIG. 7. The major difference in forming the initialcavity 144 between the preferred embodiment of the MEMS switch and thisembodiment is that the initial cavity 144 is now separated into three(3) distinct cavities corresponding to the cavities 112, 114 and 116depicted in FIG. 3. The walls 154 and 156 which have openings in thepreferred embodiment as depicted in FIG. 6 are now continuous, thusseparating the initial cavity 144 into three separate cavities. The nowburied conductors 226 carry the electrical signals under the walls 154and 156. Then, similar to the alternative embodiment illustrated inFIGS. 13 and 14, saw cuts 204 are made in the base wafer 104 along rowsof the cavities 252 thereby exposing the bonding pads 248 isolatedtherein. Subsequent sawing completely through the combined base wafer104, device layer 122, glass substrate 174 and support wafer 232 yieldsthe individual MEMS switches.

FIG. 20 depicts one cavity 252 with bonding pads 248 located therein,vias 242 passing through the glass substrate 174, and the conductors 226within the channels 222. The illustration of FIG. 20 also shows anelectrical lead 198 wire bonded to one of the bonding pads 248.Alternatively, solder bumps may be formed on the bonding pads 248.

INDUSTRIAL APPLICABILITY

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is purely illustrative and is not to be interpreted aslimiting. For example, while a single crystal silicon layer for formingthe seesaw 52 is preferably the device layer of a SOI wafer, it may alsobe an N-type top layer of epi on an epi wafer. While material of thedevice layer 122 to which ends of the torsion bars 66 a and 66 bfurthest from the seesaw 52 are coupled forms a frame which preferablysurrounds the seesaw 52, the seesaw 52 of a MEMS switch in accordancewith the present invention need not be surrounded by material of thedevice layer 122. While metallic conductors included in the MEMS switchare preferably gold (AU) applied to a Titanium (Ti) adhesion layer, theycould be made using any number of other material combinations such asplatinum (Pt) on titanium (Ti) or tungsten (W). The metals may beapplied by any of the common deposition methods used in semiconductorprocessing, which include sputtering, e-beam deposition and evaporation.

There also exists an alternative to using electrical leads 198 connectedto contact pads and grounding pads 186 for coupling signals into and outof the MEMS switch. Because the base wafer 104 can be thinned to athickness of less than 100 microns, electrical signals can alternativelybe coupled into and out of the MEMS switch using solder bumps formed onthe contact pads and grounding pads 186. The presence of solder bumps onthe contact pads and the grounding pads 186 permits flip-chip attachmentof the MEMS switch to mating solder bumps present on a printed circuitboard.

Similarly, while the preferred embodiment MEMS switch disclosed hereinis a single-pole double-throw (“SPDT”) switch, it may be readily adaptedfor construction as two, mutually exclusive single-pole single-throw(“SPST”) switches. These two mutually exclusive SPST switches may thenconfigured to operate as a SPDT switch by properly connected wiring thatis outside the MEMs switch. Furthermore, instead of the switch contacts56 a 1, 56 a 2, 56 b 1 and 56 b 2 and the two shorting bars 58 a and 58b, a SPDT MEMS switch in accordance with the present invention may beconstructed with only the switch contacts 56 a 1 and 56 b 1 and with thetwo shorting bars 58 a and 58 b being electrically connected to eachother by a conductor that is located on the seesaw 52. In such aconfiguration for the MEMS switch, the conductor which electricallycouples together the two shorting bars 58 a and 58 b on the seesaw 52connects to the common terminal 182 by an extension thereof whichtraverses one of the torsion bars 66 a and 66 b.

Moreover, more than one seesaw 52 together with its associatedelectrodes 54 a and 54 b and switch contacts 56 a 1, 56 a 2, 56 b 1 and56 b 2 may be incorporated in a single MEMS switch in accordance withthe present invention. Using two seesaws 52 with their associatedelectrodes 54 a and 54 b and switch contacts 56 a 1, 56 a 2, 56 b 1 and56 b 2 it is possible to provide a single-pole four-throw (SP4T) MEMSswitch. While external wiring may configure a MEMs switch in accordancewith the present invention to operate as a shunt switch, the MEMS switchitself can be configured to operate as a shunt switch by connecting theshorting bars 58 a and 58 b to ground. In such a shunt switch, theswitch contacts 56 a 1, 56 a 2, 56 b 1 and 56 b 2 could be a continuousconductor lacking the gap appearing therein FIGS. 1 and 9.

Consequently, without departing from the spirit and scope of theinvention, various alterations, modifications, and/or alternativeapplications of the invention will, no doubt, be suggested to thoseskilled in the art after having read the preceding disclosure.Accordingly, it is intended that the following claims be interpreted asencompassing all alterations, modifications, or alternative applicationsas fall within the true spirit and scope of the invention.

1. An integral micro-electro mechanical systems (“MEMS”) switch adaptedfor selectively coupling an electrical signal present on a first inputconductor connected to the MEMS switch to a first output conductor alsoconnected to the MEMS switch, the MEMS switch comprising: a. amonolithic layer of material having micro-machined therein: i. a seesaw;ii. a pair of torsion bars that are disposed on opposite sides of andcoupled to the seesaw, and which establish an axis about which theseesaw is rotatable; and iii. a frame to which ends of the torsion barsfurthest from the seesaw are coupled, the frame supporting through thetorsion bars the seesaw for rotation about the axis established by thetorsion bars; iv. an electrically conductive first shorting bar carriedat an end of the seesaw distal from the rotation axis established by thetorsion bars; b. a base that is joined to a first surface of themonolithic layer; c. a substrate that is bonded to a second surface ofthe monolithic layer which is distal from the first surface thereof towhich the base is joined, the substrate having formed thereon: i. afirst electrode which is juxtaposed with a surface of the seesaw that islocated to one side of the rotation axis established by the torsionbars, application of an electrical potential between the first electrodeand the seesaw urging the seesaw to rotate in a first direction aboutthe rotation axis established by the torsion bars; ii. a first pair ofswitch contacts adapted to be connectable respectively to the firstinput conductor and to the first output conductor, and which: (1) aredisposed adjacent to but spaced apart from the first shorting bar whenno force is applied to the seesaw; (2) when no force is applied to theseesaw are electrically insulated from each other; (3) the firstshorting bar contacts upon application of a sufficiently strong force tothe seesaw which urges the seesaw to rotate in the first direction aboutthe rotation axis established by the torsion bars; and (4) firstelectrical conductors that respectively carry electrical signals betweenthe switch contacts and the first input and first output conductors; andd. a first ground plate which is disposed adjacent to and iselectrically insulated from the first electrical conductors; wherebyupon rotation of the seesaw about the rotation axis established by thetorsion bars in the first direction to such an extent that the firstshorting bar contacts the first pair of switch contacts, the contactingfirst shorting bar electrically couples together the first pair ofswitch contacts.
 2. The MEMS switch of claim 1 that is further adaptedfor selectively coupling an electrical signal present on a second inputconductor connected to the MEMS switch to a second output conductor alsoconnected to the MEMS switch: wherein the seesaw carries a secondshorting bar at an end of the seesaw that is located on an opposite sideof the rotation axis from the first shorting bar; and wherein thesubstrate also has formed thereon: iii. a second pair of switch contactsadapted to be connectable respectively to the second input conductor andto the second output conductor, and which: (1) are disposed adjacent tobut spaced apart from the second shorting bar when no force is appliedto the seesaw; (2) when no force is applied to the seesaw areelectrically insulated from each other; (3) the second shorting barcontacts upon application of a sufficiently strong force to the seesawwhich urges the seesaw to rotate in a second direction about therotation axis established by the torsion bars that is opposite to thefirst direction; and (4) second electrical conductors that respectivelycarry electrical signals between the switch contacts and the secondinput and second output conductors; and e. a second ground plate whichis disposed adjacent to and is electrically insulated from the secondelectrical conductors; whereby upon rotation of the seesaw about therotation axis established by the torsion bars in the second direction tosuch an extent that the second shorting bar contacts the second pair ofswitch contacts, the contacting second shorting bar electrically couplestogether the second pair of switch contacts.
 3. The MEMS switch of claim2 wherein the substrate also has formed thereon a second electrode whichis juxtaposed with a surface of the seesaw that is located to one sideof the rotation axis established by the torsion bars which is oppositeto the surface of the seesaw with which the first electrode isjuxtaposed, application of an electrical potential between the secondelectrode and the seesaw urging the seesaw to rotate in the seconddirection about the rotation axis established by the torsion bars. 4.The MEMS switch of claim 1 that is further adapted for selectivelycoupling an electrical signal present on a second input conductorconnected to the MEMS switch to the first output conductor: wherein theseesaw carries a second shorting bar at an end of the seesaw that islocated on an opposite side of the rotation axis from the first shortingbar; and wherein the substrate also has formed thereon: iii. a secondpair of switch contacts a first one of which is adapted to beconnectable respectively to the second input conductor and a second oneof which is connected to that one of the second pair of switch contactswhich is adapted to be connectable to the first output conductor, andwhich: (1) are disposed adjacent to but spaced apart from the secondshorting bar when no force is applied to the seesaw; (2) when no forceis applied to the seesaw are electrically insulated from each other; (3)the second shorting bar contacts upon application of a sufficientlystrong force to the seesaw which urges the seesaw to rotate in a seconddirection about the rotation axis established by the torsion bars thatis opposite to the first direction; and (4) second electrical conductorsthat respectively carry electrical signals between the switch contactsand the second input and first output conductors; and e. a second groundplate which is disposed adjacent to and is electrically insulated fromthe second electrical conductors; whereby upon rotation of the seesawabout the rotation axis established by the torsion bars in the seconddirection to such an extent that the second shorting bar contacts thesecond pair of switch contacts, the contacting second shorting barelectrically couples together the second pair of switch contacts.
 5. TheMEMS switch of claim 4 wherein the substrate also has formed thereon asecond electrode which is juxtaposed with a surface of the seesaw thatis located to one side of the rotation axis established by the torsionbars which is opposite to the surface of the seesaw with which the firstelectrode is juxtaposed, application of an electrical potential betweenthe second electrode and the seesaw urging the seesaw to rotate in thesecond direction about the rotation axis established by the torsionbars.
 6. The MEMS switch of claim 1 wherein a fusion bond joins themonolithic layer and the base.
 7. The MEMS switch of claim 1 whereinmaterial forming the monolithic layer is single crystal silicon (Si). 8.The MEMS switch of claim 1 wherein a sheet of electrically insulatingmaterial is interposed between the seesaw and shorting bar.
 9. The MEMSswitch of claim 1 wherein the base includes a cavity formed thereinwhich abuts the first surface of the monolithic layer, and into which aportion of the seesaw enters upon rotation of the seesaw about the axisestablished by the torsion bars.
 10. The MEMS switch of claim 1 whereinthe ground plate is disposed on the monolithic layer.
 11. The MEMSswitch of claim 10 wherein the monolithic layer includes a cantileverwhich supports at a free end thereof a grounding island which at an endthereof which is distal from the cantilever carries a portion of theground plate, the portion of the ground plate at the end of thegrounding island being continuously urged by force supplied by thecantilever into an unswitched, intimate contact with an electricalconductor that is disposed on the substrate.
 12. A micro-electromechanical systems (“MEMS”) structure comprising: a first layer havingdisposed thereon an electrical conductor; and a second layer also havingdisposed thereon an electrical conductor, the second layer including: a.a cantilever; and b. an electrical contact island which is supported ata free end of the cantilever, the electrical contact island at an endthereof which is distal from the cantilever carrying a portion of theelectrical conductor that is disposed on the second layer, the portionof the electrical conductor at the end of the electrical contact islandbeing continuously urged by force supplied by the cantilever into anunswitched intimate contact with the electrical conductor that isdisposed on the first layer.
 13. A micro-electro mechanical systems(“MEMS”) electrical contact structure adapted for forming an unswitchedelectrical contact between an electrical conductor that is disposed on afirst layer of a MEMS device and an electrical conductor that isdisposed on a second layer of the MEMS device, the MEMS electricalcontact structure comprising: a cantilever included in the second layer;and an electrical, contact island also included in the second layerwhich is supported at a free end of the cantilever, the electricalcontact island at an end thereof which is distal from the cantilevercarrying a portion of the electrical conductor that is disposed on thesecond layer, the portion of the electrical conductor at the end of theelectrical contact island being continuously urged by force supplied bythe cantilever into an unswitched intimate contact with the electricalconductor that is disposed on the first layer.
 14. An integralmicro-electro mechanical systems (“MEMS”) switch adapted for selectivelycoupling an electrical signal present on a first input conductorconnected to the MEMS switch to a first output conductor also connectedto the MEMS switch, the MEMS switch comprising: a. a monolithic layer ofmaterial having micro-machined thereon a moveable electricallyconductive first shorting bar that is disposable in at least two (2)alternative positions; b. a base that is joined to a first surface ofthe monolithic layer; c. a substrate that is bonded to a second surfaceof the monolithic layer which is distal from the first surface thereofto which the base is joined, the substrate having formed thereon a firstpair of switch contacts which: i. are disposed adjacent to but spacedapart from the first shorting bar when the first shorting bar isdisposed in a first position; ii. are electrically insulated from eachother when the first shorting bar is disposed in the first position;iii. the first shorting bar contacts when the first shorting bar isdisposed in a second position; and iv. connect to a pair of firstelectrical conductors that are respectively adapted for conducting anelectrical signal between the first pair of switch contacts and thefirst input and first output conductors; and d. a first ground platewhich is disposed adjacent to and is electrically insulated from both:i. the first pair of switch contacts; and ii. the first electricalconductors; whereby movement of the first shorting bar from the firstposition to the second position establishes an electrical connectionbetween the first shorting bar and the first pair of switch contactsthereby electrically coupling together the first pair of switch contactswhile the first ground plate remains separated from but in closeproximity to both: a. the first pair of switch contacts; and b. thefirst electrical conductors.
 15. The MEMS switch of claim 14 wherein asheet of electrically insulating material is interposed between themonolithic layer and shorting bar.
 16. The MEMS switch of claim 14 thatis further adapted for selectively coupling an electrical signal presenton a second input conductor connected to the MEMS switch to a secondoutput conductor also connected to the MEMS switch: wherein themonolithic layer carries a second moveable electrically conductiveshorting bar that is disposable in at least two (2) alternativepositions; and wherein the substrate also has formed thereon a secondpair of switch contacts which: i. are disposed adjacent to but spacedapart from the second shorting bar when the second shorting bar isdisposed in a first position; ii. are electrically insulated from eachother when the second shorting bar is disposed in the first position;iii. the second shorting bar contacts when the second shorting bar isdisposed in a second position; and iv. connect to a pair of secondelectrical conductors that are respectively adapted for conducting anelectrical signal between the second pair of switch contacts and thesecond input and second output conductors; and e. a second ground platewhich is disposed adjacent to and is electrically insulated from both:i. the second pair of switch contacts; and ii. the second electricalconductors; whereby movement of the second shorting bar from the firstposition to the second position establishes an electrical connectionbetween the second shorting bar and the second pair of switch contactsthereby electrically coupling together the second pair of switchcontacts while the second ground plate remains separated from but inclose proximity to both: a. the second pair of switch contacts; and b.the second electrical conductors.
 17. The MEMS switch of claim 16wherein the second shorting bar moves from the first position to thesecond position synchronously with movement of the first shorting barfrom the second position to the first position.
 18. The MEMS switch ofclaim 14 that is further adapted for selectively coupling an electricalsignal present on a second input conductor connected to the MEMS switchto the first output conductor: wherein the monolithic layer carries asecond moveable electrically conductive shorting bar that is disposablein at least two (2) alternative positions; and wherein the substratealso has formed thereon a second pair of switch contacts which: i. aredisposed adjacent to but spaced apart from the second shorting bar whenthe second shorting bar is disposed in a first position; ii. areelectrically insulated from each other when the second shorting bar isdisposed in the first position; iii. the second shorting bar contactswhen the second shorting bar is disposed in a second position; and iv.connect to a pair of second electrical conductors that are respectivelyadapted for conducting an electrical signal between the second pair ofswitch contacts and the second input conductor and the first outputconductor; and e. a second ground plate which is disposed adjacent toand is electrically insulated from both: i. the second pair of switchcontacts; and ii. the second electrical conductor; whereby movement ofthe second shorting bar from the first position to the second positionestablishes an electrical connection between the second shorting bar andthe second pair of switch contacts thereby electrically couplingtogether the second pair of switch contacts while the second groundplate remains separated from but in close proximity to both: a. thesecond pair of switch contacts; and b. the second electrical conductor.19. The MEMS switch of claim 18 wherein the second shorting bar movesfrom the first position to the second position synchronously withmovement of the first shorting bar from the second position to the firstposition.
 20. The MEMS switch of claim 14 wherein a fusion bond joinsthe monolithic layer and the base.
 21. The MEMS switch of claim 14wherein material forming the monolithic layer is single crystal silicon(Si).
 22. The MEMS switch of claim 14 wherein the ground plate isdisposed on the monolithic layer.
 23. The MEMS switch of claim 22wherein the monolithic layer includes a cantilever which supports at afree end thereof a grounding island which at an end thereof which isdistal from the cantilever carries a portion of the ground plate, theportion of the ground plate at the end of the grounding island beingcontinuously urged by force supplied by the cantilever into anunswitched, intimate contact with an electrical conductor that isdisposed on the substrate.